This invention relates to a process for the hydroxyalkylation of polysaccharides.
In commercial processes starch is hydroxyethylated by the following procedure. A 0.5 percent by weight slurry of starch in water is contacted with acid to digest the starch so as to lower the molecular weight of the starch. Thereafter, the slurry is treated with a strong base, such as sodium hydroxide, to make the slurry basic. The slurry is then contacted with ethylene oxide and reacted for between about 17 and 22 hours at 40.degree. C. Thereafter the slurry is neutralized. The hydroxyethylated starch can be removed from the water by filtration. The acid digestion step results in a product with a lower molecular weight than the original starch reactant. Ethylene oxide has the tendency to undergo polyethoxylation, making it difficult to prepare a monoethoxylated product. Furthermore, a large percentage of the ethylene oxide forms ethylene glycol as a by-product.
Srivastava et al., Indian Journal of Chemistry, 9, 1081 (1971), disclose three processes for hydroxyethylating glucose, starch and amylopectin. In one process described, D-glucose is hydroxyethylated with ethylene carbonate with sodium hydroxide as the catalyst. It is reported that the process gives poor hydroxyethylation efficiency. It is also disclosed that the use of metallic sodium as a catalyst and a polar solvent such as dimethylformamide gave satisfactory results at temperatures above 100.degree. C. for the hydroxyethylation of glucose. The best process, according to the author, uses sodium hydride as a catalyst and dimethylsulfoxide as a solvent. In this latter process a mixture of the reactant and the product is recovered from the reaction solution by extraction with chloroform and evaporation of the chloroform, or by precipitation with acid and filtration.
What is needed is a process for hydroxyalkylation of polysaccharides in which polyalkoxylation is avoided. A process in which the polysaccharide is not digested by contact with acid is desirable. A process in which the hydroxyalkylated polysaccharide can be easily separated from the reaction mixture is also desirable.